With modern motor vehicle transmissions, in particular automatic transmissions, gears are shifted by shifting elements, i.e. clutches or brakes, designed as multi-disk shifting elements or claw shifting elements. Usually, the shifting takes placing hydraulically, i.e. through shifting devices in the form of piston/cylinder units, which are coated with pressurized oil. The pressurized oil is conveyed by a pump and through oil supplies of the transmission housing through so-called rotating oil feeds, into the rotating gear shaft, and led from it to the shifting devices. Losses that burden the transmission efficiency arise from the hydraulic circuit, i.e. the performance of the transmission oil pump, seals, pressure drops in the supply lines, and leakage. The problem with this system is that losses caused by the hydraulic circuit, in particular by shifting, must be kept as low as possible, in order to achieve the highest transmission efficiency possible. A particular problem is that, in a closed shifting element, the oil pressure in a hydraulic cylinder, the so-called closing pressure, must be constantly tracked, i.e. maintained, in order to be able to deliver the necessary turning moment into the shifting element. This pressure must be applied by a hydraulic pump, which is driven by the engine of the motor vehicle, which brings about losses.
A hydraulically shiftable multi-disk shifting element is known from DE 102 05 411 A1 of the applicant, wherein in a closed state, i.e. upon the transmission of the turning moment, the multi-disk shifting element is mechanically locked by a locking device. Thereby, the contact force between the multi-disks is maintained, without hydraulic pressure having to act—the transmission oil pump is thereby unburdened. In a variant that is not shown in the reference, the locking device is designed as a shut-off valve, which is arranged in the supply area of the hydraulic cylinder. Thus, the pressure space of the cylinder at the pump is shut off, and the necessary pressure is maintained for maintaining the contact pressure in the multi-disk pack. Thus, with this variant as well, if the shifting element is closed, the transmission oil pump is unburdened.
A synchronization for a manual transmission is known from DE 10 2006 049 283 A1 of the applicant, whereas an idler gear is able to be shifted by means of an actuator arranged in a gear shaft, designed as a cylinder/piston unit. The cylinder of the actuator features two pressurized medium lines, in which shut-off valves are arranged, through which the pressure prevailing in the cylinder can be maintained, without the pump having to consume power.